Refrigeration



Ufff 4, 1943# v E. Lomfvn 2,318,318

` REFRIGERTION Filed May 23, 1942 ,2 Sheets-Sheet l JJ? 45 RNVENTOR Rw/N oww/a EMA., jD/W, MM r Lw 4g w ATTORNEYS May 4 1943- E. LoDwlG 2,318,318

REFRIGERATION y Filed May 23, 1942 2 Sheets-Sheet 2 MOTOR 5741? TER INVENTOR EfW//V L OOWIG Patented May 4, 1943 UNITED sTATl-:s PATENT OFFICE@ 2,318,311; nemica-narici( Erwin mwa, AFranklin square, N. Y., am to Mobile Refrigeration, Inc.,l New York, N. Y.,

12 Claims.

temperature variation encountered in flight' a corporation of'New .York Application May 23, 1942, Serial No. 444,229

and the rapidity with which these changestake! v' place necessitate the use of reirigeratlng equipment of considerable capacity in order to produce the conditions necessary for testing equipnient used in flight. Chambers for use in the described operation must be of considerable mass order vto withstand the wide variations in niiiamber air pressure used to simulate flight renditions, and 'the mass of the chamber may considerably increase the heat load imposed on the refrigerating equipment. In testing or deveiopment work of this natu-re, the air within the chamber must frequently be cooled rapidly through temperature ranges of the vorder of 500 F. or more. i Y

It is the object of the present invention to `provide an improved refrigerating system for rapidly and eiilciently lowering the temperature ni the air or other gas in a closed compartment cr chamber. The system is also applicable to the cooling of other uids or objects. In general,

the refrigerating system of the invention is of 'the compression type and includes a plurality ci compressors, and means are providedvfor altering the connection of the compressors under the control of the 4pressure difference through which they compress the refrigerant gas in such fashion as to provide parallel operation of the compressors when the pressure difference is low and series or compound operation thereof when the pressure difference is high. This arrangement combines high displacement of refrigerant aas by the compressors and consequent rapid decrease ln the chamber temperature-over the higher temperature range, and high volumetric efficiency of the compressorsover the lower temperaturel range oi!- operation.

Other objects, features. and advantages of the l invention will be apparent from the following description of certain` typical embodiments thereof. In describing theinvention in detail, reference will be made to the accompanying drawings. in which Fig. 1 is a diagrammatic and slmplied representation of a. refrigerating system embodying the invention and connected to cool a closed'test chamber; and

' Fig. 2 is a similar representation of a modified form of refrig'erating system embodying the invention.

Referring to Fig. 1, I have represented at C a chamber for use in testing or developing aircraft instrinnents or devices' under varying conditions of pressure and temperature, such conditions including rapid drops in temperature over considerable ranges. The chamber is closed during operation and the temperature of the air therein is lowered by suitable coolingunits or evaporators. -In the illustrated embodiment, an evaporator coil 3 is provided within the chamber, and in addition the chamber walls, five of which are shown, act. as evaporators or cooling units. This may be accomplished by employing hollow walls as shown at 4, I, 6 and 1, and expanding refrigerant within these walls and within a similar hollow rearvwali 8. The use of the chamber walls as evaporators insures that heat will be rapidly removed from Athe chamber wall structure at the same time that the chamber air is being cooled by the evaporator coil 3.

Since chambers of the nature illustrated must be substantially completely evacuated during operation, the chamber walls are necessarily somewhat massive and the rapid cooling of the walls is necessary if the chamber air temperature is to berapidly reduced. stood that the chamber walls may be insulated from the surrounding atmosphere by any suitable known means.

The refrigerant expanded in the evaporator comprising the coil 3 and the hollow chamber walls 4, 5, 6, l and `8 is supplied by a compression type refrigerating system. The system includes a. receiver 9 from which liquid refrigerant is fed through 'a duct Ill, a solenoid valve 52, an expension valve II, and a refrigerant feed duct i2 to the evaporator coil I, and through ducts I U and i3, a solenoid valve 53, an expansion valve i4 and feed ducts i5, i8, i1, I8 and i9 to the chamber wall evaporator-s 1. l, 5, l and 6, reA spectively. 'I'he refrigerant return duct 2li from the expansion coil 3 enters a manifold 2|, and the return ducts 22, 2l, 24, 2l and 26 from the several chamber wall evaporators communicate with s. return manifold 21. The two manifolds It will be under- 2| and 21 are connected as shown to a common suction line 28.

The ilowof refrigerant to the evaporator coil 3 is thermostatically controlled in accordance with changes in the temperature of the expanded refrigerant returning from the coil to the manifold 2|. The operation of the expansion valve is governed by a temperature responsive bulb or element 23 in the manifold 2| in such fashion as to increase the feed of refrigerant to the coil when the temperature of thereturned refrigerant rises above a predetermined value, and to reduce the refrigerant feed when the returned refrigerant temperature falls below a predetermined value. The expansion valve I4 similarly controls the flow of refrigerant to the chamber wall evaporators 4, 5, 8, 1 and 3 and is similarly thermostatically controlled in accordance with changes in the temperature of the expanded refrigerant in the manifold 21 by the bulb or element 38. Since thermostatically controlled expansion valves of this nature are well known in the art, a detailed description of the valves I and I4 and their control elements 29 and/30 will be omitted to simplify the disclosure.

Referringl now to the compressing and condensing unit which withdraws expanded refrigerant from the suction line 28 and supplies liquid refrigerant to the receiver 5, this includes at least two compressors 3| and 32 and a condenser 33. I'

Various forms of compressors may be used, and in the embodiment of Fig. 1, two reciprocating compressors are shown. The compressor 3| preferably has a larger volumetric capacity than the 'compressor 32. It should be understood that single or double acting compressors of single or multiple cylinder construction may be used. The compressors 3| and 32 are driven by individual electric motors 40 and 34, respectively, but a single motor or other prime mover may be used to drive both compressors if desired.

The suction line 23 is connected to the intake of the compressor 3| through a duct 35 and to the intake of the compressor 32 through/ a duct 33 having a check valve 31 therein. The valve 31 permits the ow of refrigerant from the ,suction line 23 to the compressor 32, but prevents return flow-of refrigerant from the compressor 32 to the line 23 or. to the intake of thecompressor 3 I. The

ydischarge of the compressor 3| is connected by conventional control means. As shown, a temperature responsive bulb or` element 50 is provided in the chamber C. When the temperature of the air in the chamber C rises above a predetermined value (which is preferably adjustable) the element-C closes a control switch of known construction, and this switch when so closed energizes and opens the solenoid valves 52 and 53 in the refrigerant feed lines to the evaporator coil 3 and the chamber wall evaporators, respectively. Energy for the solenoid valves 52 and 53 is supplied from any suitable source, indicated at 54, When the chamber air temperature is reduced to the required value, the temperature responsive element 50 causes the switch 5| to open and so de-energizcs and closes the solenoid valves 52 and 53, cutting oif further supply of refrigerant to the evaporators.

If desired, heat economies may be 'effected by heat exchange between the refrigerant supply line I0 and the return line 20 and between the supply line1|3 and the return manifold 21. Heat exchanging means for this purpose is conventionally illustrated at 55 and 56.

The compressor motors 40 and 3f. are electrically connected in parallel as shown and are energized to operate the compressors by a motor starter under control of the suction line pressure. To this end, a pressure operated switch Pi is connected through a pipe 46 to the suction line 28 and acts through an expansible bellows" to close its contacts 48 when the suction line pressure rises above a predetermined value. Closure of the contacts 48 e'nergizes the motor starter 45 which then acts in a known manner to energize the compressor drive motors 40 and 34 from a suitable source of energy connected to the leads 49. When the refrigerant pressure in the suction line 28 falls below a predetermined value, the

contacts 48 of the device Pr open and the compressor motors are de-energized by the motor starter 45.

The solenoid valve 44 in `the by-pass duct 42 is controlled by a pressure operated switch Pz.

The bellows 5B ofc-this switch 1s connected to the suction. line l23. Thearrangement is such that when the refrigerant pressure in the suction line the duct 33. The discharge of the compressor 32 is connected to the inlet of the condenser 33 through the duct 4|. A by-pass duct 42 including an inter-cooler 43 of known construction-is nected between the discharge of the compressor 3| and the inlet of the compressor 32. A Vsolenoid valve 44 is provided in the by-pass duct 42 and is arranged to open when its solenoid is energized and to close when its solenoid is de-energized.

The compressor 3| may be cooled by any suitable medium such as air or water. The condenser 33 is designed with excess heat absorbing capacity so that the condensing pressure of the refrigerant is substantially constant under all loads with\I in the capacity of the system. With this arrangement, the compressors 3| and 32 compress the A"refrigerant against a substantially constant maximum 'condensing pressure, and the range through which the refrigerant is compressed varies with falls below a predetermined value, the bellows 53 Vcloses the contacts 53 of the switch Pz, energizing and so opening the solenoid valve 44 through a circuit that includes a suitable source of energy connected at 51. rWhen the pressure of the re; frigerant in the suction line rises above the predetermined value, the bellows 58 opens the contacts 5,9 and the solenoid valve 44 is de-energized and closed.

' In the operation of the system of Fig. 1, when the chamber air temperature rises above the value for which the control switch 5| is set, thisI switch energizes and so opens the solenoid valves 52 and 53, admitting the refrigerant from the receiver 3 to the Yevaporators 3, 4, 5, 5, 1 and 8 under control of the expansion valves Il and I4. The valves 0 II and I4 control the flow of refrigerant to the lvery low, say four inches of mercury absolute, but the evaporating refrigerant rapidly increases this pressure. When the suction line pressure has increased to a predetermined value, say six inches of mercury absolute, the switch P1 closes its contacts 4I and the motors 40 and 34 are energized, starting operation of the compressors 3| and 32. At this low suction line pressure, the contacts 59 of the switch Pz are closed and the solenoid valve 44 is accordingly open. Assuming a, considerable heat load on the chamber, such as is encountered in cooling the chamber from room .temperature or above. the suction line pressure rises,V very rapidly and at a predetermined higher value, say twenty inches of mercury absolute, the switch Pa opens its contact 59, de-energizing and so closing the solenoid valve 44 in the by-pass duct 42. The 7 compressors then operate in parallel, the compressor Il drawingrefrigerant from the line 28 through theduct 35 and delivering compressed refrigerant through the duct 38 to the condenser 3l. The compressor. draws refrigerant from the line 28 through the duct 36 and discharges refrigerant through the duct 4i to the condenser l2. This parallel operation of the condensers delivers a large volume of refrigerant, compressed over a relatively low range, to the condenser 33, and so rapidly replenishes the liquefied refrigerant being withdrawn from the receiver 9 to the levaporators'. t

The evaporation of refrigerant in the evaporf ators rapidly lowers the temperature of the chamber wallsand the air within the chamber and as the temperature falls, the refrigerant pressure in the suctionline 28 is reduced. The drop in suction line pressure with constant condensing pressure in condenser 23 increases the pressure range over which the compressors operate. As thisv compression `range increases the volumetric eificiency of the compressors is reduced. When the suction line pressure has fallen to a predeter -mined low value,such that a material reduction `in suction stroke charge and hence a material re- .duction in the volumetric eiiiciency of the compressors has occurred, the switch Pa closes its contacts 56 and energizes and so opens thevr solenoid valve 44. The suction line pressure atwhich the -ewitch Pn closes its contacts may be, for example, 4about eighteen inches of mercury -absoiiite =pening of the solenoid valve 4t connecte the compressors 3i and 22 in series or compound relation. The larger compressor 3i draws refriger- 'ant from the suction line 26 through the duct 3, compresses this refrigerant through a portion or the compression range and delivers it through. the Lduct 36 and the by-pass duct 42 and inter-cooler W3 to the intake of the compressorru. The cons'equent'rise in pressure 'at the intake of the :coin- -'pressor- 32 closes the check valve 3i.f The com- 'pressor 32 receives refrigerant from the icy-pass lduct 42 and compresses it'to the condensing pressure, delivering the compressed refrigerant through the duct 4I to the condenser It. The increased pressure at the condenser inlet closes the check valve 36. l

The change to compound or series connection of the compressors reduces the pressure range through which each compressor operates, and so increases the-volumetric efiiciency o! the compressors. 'I'hs enables the system to produce very low temperatures in the chamber without either employing excess refrigerating capacity or reducing the rate at which the chamber temperature is lowered over the higher temperature ranges. In other words, with the systemof the mercury absolute, whereupon the control switch pressors in parallel or in series is automatically y governed by the compression range through which they-operate. as determined by the pressure in the common suction line connected to the evaporators.

When the chamber temperature is reduced to ,i the desired value, the control switch i de-energizes and so closes solenoid valves 52 andv 53. This cuts oil' further flow of refrigerant to the evaporators, and as soon as the refrigerant remaining therein has evaporated, the compressors reduce the suction line and evaporator pressure to a minimum value,.say about four inches of vPi opens its`contacts 48 and the compressor `condenser "it through a duct 32.

motors 40 and 34 are (le-energized, thus completing the operating cycle.

Any desired number of compressors may be used in myV improved system. In Fig. 2 I have illustrated an embodiment in which three com# pressors are used. The common suction line connected to the evaporating means is shown at 28' and it is connected to the intake of .a compressor 6D of large displacement through a duct 6|. A duct 62 with a check valve 63 therein connects the suction line with the intake as a compressor 64 of intermediate displacement, and a duct 65 with a check valve 66 therein connects the duct 62 with the intake of a third com pressor 61 of relatively small displacement. As

in the embodiment of Fig. 1, each of the compressors 60, $4 and 61 may be operated by a motor til, 69 andv 10, and the motors are shown arranged to be energized in parallel from a suitable source 'ii by a motor starter 12 under con `trol of a switch Pa, operated by the suction line pressure. The compressor Gli discharges through a duct 'lite the inlet oi the condenser 14 and a check valve l5 prevents return now through the duct lli.` n by-pass pipe It including an inter- -cooler il connects the discharge of the compressor tt tothe inlet of the compressor 64, and a solenoid valve Si controls this by-pass. The valve Si is opened when energized and is con- ,itrolleci by a switch P4 in accordance with the suction line pressure.

The discharge' of the intermediate compressor Fit in connected to the condenser inlet through a duct 'il having a check valve i9 therein. A bypass duet including an inter-cooler 8| con ,meets the discharge of they compressor with the inlet of the compressor 6l. A solenoid valve te is provided in the duct 80 and is controlled by a pressure operated switch Ps responsive to the pressure in the suction line 26'. The dis'- charge oi the compressor 51 is connected to the V As in the embodiment oi Fig. l, the condenser 14 has s'uicient cooling capacity so that it operates at substantiaily constant condensing pressure. A receiver i9 receives liquid refrigerant from the condenser 1t. The. liquid refrigerant may be delivered to any suitable evaporator or evaporators Vsuch as s those employed to cool a chamber, as disclosed above in connection with Fig. 1.

The three compressors of the system of Fig. 2

'75 operate in parallel under lower ratios of suction' line pressure to condenser pressure.` When this pressure ratio increases above a predetermined value, the first compressor lllv is connected in series with the intermediate and final compressors M and l1 in parallel. When the compression range between the suction line and the condenser reaches a higher predetermined value, the three compressors are connected in series.

When the compressors are started by closure of the pressure operated switch Pa in response to a rise in suction line pressure from the minimum value, the suction line pressure rises rapidly, as explained above in connection with the ernbodiment of Fig. 1, anc when this pressure reaches a value of say twenty inches of mercury absolute, the switches P4 and Ps are both opened and hence the solenoid valves Si and Sn are both de-energized and closed. The compressors B0, I4 and Il then operate in parallel` and compress the expanded refrigerant at high volume through a relatively low pressure range. As the suction line pressure is reduced, a point is reached where the volumetric efficiency of the compressors falls off, and the pressure operated switch Po closes, energizing and so opening the solenoid valve Si. This may occur, for example, at a' suction line pressure oi' about eighteen inches of mercury absolute. When the solenoid valve Si opens, the vcompressor 60 discharges through the by-pass discharge pressure of thecompressor 60 closes v the check valve 1L 'Ihe compressor 80 now acts as a' iirst compression stage, compressing the refrigerant over a reduced pressure range, and

the compressors N and 61 act in parallel as a second compression stage, compressing the refrigerant over the remainder ofxthe compression range. 'I'hev result is that the volumetric ei'il ciency of the compressors is increased by the reduction in the ranges through which they operate.

As the suction line pressure continues to drop,

' the volumetric eiilciency of the compressors again falls, and at a predetermined low suction line Pressure, say about thirteen inches of mercury absolute, the switch Ps closes and so energizes and opens the solenoid valve Sz. This connects the compressors Il and l1 in series, the refrigerance with the suction line or evaporator pressure for maximum volumetric emciency.

pressures at which the various control devices operate have been recited herein by way of example only. and that the invention is in no sense limited to theY particular values noted.

I claim: v

1. In a re'frigerating system, in combination with a refrigerant evaporator, a refrigerant condenser and means for conducting condensed refrigerant from said condenser to said evaporator, at least two compressors connected in parallel between said evaporator and said condenser, and means responsive to al predetermined increase in the difference in pressure` between the refrigerant in said evaporator and the refrigerant in said condenser for connecting said compressors in series between said evaporator and said condenser.

2. In a refrigerating system, in combination with a refrigerant evaporator, a refrigerant con? denser and means for conducting condensed re` frigerant from said condenser to said evaporator,

' at least two compressors for compressing refrigerant withdrawn from said evaporator and' delivering compressed refrigerant to said condenser, and. means ,responsive to the pressure of the re- `frigerant withdrawn from said evaporator for connecting said compressors in' parallel when said pressure is above a predetermined value and for connecting lsaid compressors in series when said pressure is below said value.

3. In a refrigerating system, in combination with a refrigerant evaporator, a refrigerant condenser and means for conducting condensed refrigerant from said condenser to said evaporator,

sure and delivering such refrigerant to said conant discharged from the compressor Il passing through the by-pass duct .l and inter-cooler Il to the inlet of the compressor 61. The increase in pressure at the inlet of the compressor l1 closes the check valve CI, and the drop in discharge pressure of .the compressor I4 relative to the condenser pressure closes the check valve 1l. I'he three compressors Il, u and I1 then act 'as first, second, and third compression stages, respectively, and since the pressure range through which each compressor operates is reduced, the volumetric eillciency of each is increased.

It' should be noted that in both ofthe mustrated embodiments of the invention. the connection of the compressors is determined by the ratio of suction line pressure to condenser pressure. .'I'hus, if the compression unit starts operating when the temperature of the refrigerated spaceis relatively low andthe suction linepreslure accordingly remains relatively low, `the ew-iiloperateinseriesorinseriesparallolrelation. -Inanycase,theconnection.of the is automatically adjusted in accordn denser, and means responsive to changes in the evaporator pressure for connecting said compressors in parallel when said evaporator pressure is i above a predetermined value and connecting said compressors in series when said evaporator pressure is below a predetermined value. .f

4.1Iri a refrigerating system, in combination with a refrigerant evaporator, a refrigerant condenser and means for conducting refrigerant from said condenser po said evaporator, a. compression unit comprising at least two compressors each having an intake and a discharge, a suction lino for withdrawing evaporated refrigerant from said evaporator. ducts vconnecting said suction line to the intakes lof said compressors in parallel, ducts for conducting compressed refrigerant from the. discharges of the respective compressors to said condenser. a lay-pass duct connected between the discharge of one of said compressors and the intake of another of said compressors, means for `preventing return now of refrigerant from .said

condenser to the compressor discharge to which said by-pass duct is' connected, means for preventing returnflow of refrigerant from the compresser intake vto whichsaid -by-nass duct is connectedto said suction line, a valve in said by-pass duct.; and means responsive to the refrigerant pressure in said suction line for opening said valve when said pressure is below a predetermined -value and closing said valve when said pressure is above saidpredetermined value.

5. In a refrigerating system, in combination with a refrigerant evaporator, a refrigerant condenser and means for conducting refrigerant from said condenser to said evaporator. a compression unit comprising at least two compressors connected to withdraw evaporated refrigerant from said evaporator and deliver compressed refrigerant to said condenser, means responsive to the refrigerant pressure in said evaporator for starting operation of said compressors when said pressure rises above a rst predetermined value and for discontinuing operation of said compressors when said pressure falls below said predetermined value, and further means responsive to the refrigerant pressure in said evaporator for connecting said compressors in series when said pressure is below a second predetermined value higher than said first predetermined value and for connecting saidcompressors in parallel when said pressure is above said second predetermined value.

9. In a refrigerating-` system, in combination i with a refrigerant evaporator, a refrigerant condenser and means for conducting refrigerant from 6. In a refrigerating system, in combination with a refrigerant evaporator, a refrigerant condenser andmeans for conducting refrigerant from said condenser to said evaporator, a compression unit comprising two compressors connected tov withdraw evaporated refrigerant from said evaporator and deliver compressed refrigerant to said condenser, and means responsive to the difference in pressure between the refrigrant in said evaporator and the refrigerant in said condenser for connecting said compressors in parallel when said pressure difference is below a predetermined value and connecting said compressors in series when said pressure difference is above said predetermined value.

7. In a refrigerating system in combination with a refrigerant evaporator, a refrigerant condenser for condensing refrigerant at substantial- ,I

ly constant pressure and means for conducting compressed refrigerantfrom said condenser to said evaporator, a compression unit comprising two compressors of different volumetric capacity connected to withdraw refrigerant from said evaporatorand deliver compressed refrigerant to said condenser, and means responsive to the pressure of the refrigerant withdrawn from said evaporator for connecting said compressors in parallel between said evaporator and said condenser when said pressure is above a predetermined value and for connecting the compressor of larger volumetric capacity to said evaporator and the Icompressor of smaller volumetric ca-' pacity between said larger capacity compressor and said condenser when said pressure is below said predetermined value.

8. In a refrigerating system. in combination with a refrigerant evaporator, a refrigerant condenser and means for conducting refrigerant from said condenser to said,evaporator, a compression unit comprising two compressors, each having an intake and a discharge, a suction line for withdrawing evaporated refrigerant from said evaporator, ducts connecting said suction line tothe intakes of said compressors in parallel, ducts for conducting compressed refrigerant from the dis\ charges of the respective compressors to said condenser, a by-pass duct connected between the discharge of the first compressor and the intake of the second compressor, means for preventing return flow of refrigerant fromsaid condenser to said condenser tosaid evaporator, a suction line for withdrawing evaporated refrigerant from said evaporator, a compression unit comprising three compressors lconnected to withdraw evaporated refrigerant from said suction linel and deliver compressed refrigerant to said condenser, and means responsive to the difference in pressure between the refrigerant in said suction line and the refrigerant in said condenser for connecting all of said compressors in parallel when said pressure difference is below a predetermined value. `connecting one of said compressors in series with the other two in parallel when said pressure difference rises above said predetermined value, and connecting all of said compressors in series when said pressure difference rises above a higher predetermined Value.

10. In a refrigerating system, in combination with a refrigerant evaporator, a refrigerant con-'- denser and means for conducting refrigerant from said condenser to said evaporator, a compression unit comprisingthree compressors connected to withdraw refrigerant from said evaporator and deliver compressed refrigerant to said condenser, means responsive to the pressure of the `refrigerant in said evaporator for connecting all of said compressors in parallel when said pressure is above a firs't predetermined value and for connecting one 'of said compressors in series with the-other two when said pressure is-below said rst predetermined value, and further means responsive to the pressure of the refrigerant in said evaporator for connecting all of said compressors in series when said pressure is below a second predetermined value lower than said first predetermined value.

, 11. In a refrigerating system, in combination with a refrigerant evaporator, a refrigerant condenser and means for conducting refrigerant from said condenser to said evaporator, a suction line for withdrawing evaporated refrigerant from said evaporator, a compression unit comprising a large volumetric capacity compressor, an inter- .mediate volumetric capacity compressor and a small volumetric capacity compressor connected to withdraw evaporated refrigerant from said suction line and |`deliver compressed refrigerant to said condenser, means responsive to the refrigerant pressure in said suction line for connecting all of said compressors in parallel when said pressure is above a first predetermined value and for connecting said large capacity compressor in series with said intermediate capacity compressor and said small capacity compressor in parallel when said pressure is below said first predetermined value, and further means responsive to the refrigerant pressure in said suction line for connecting lsaid compressors inseries in the order oftheir volumetric capacities when said pressure falls below a second predetermined value lower than said first predetermined value.

12. In a refrigerating system, in combination with a refrigerant evaporator, a refrigerant condenser and means for conducting refrigerant from said condenser to said evaporator, a suction line for withdrawing evaporated refrigerant `from said evaporator, a compression unit comprising three compressors each having an intake and a discharge, ducts connecting the intakes of all of said compressors to said suction line. ducts connecting the discharges of all of said compressors to said condenser, a first by-pass 'duct connecting the discharge of the iirst of said compressors to the intake of the second of said compressors, a second by-pass duct connecting -the discharge of the second of said compressors to the intake of the third of vsaid compressors, a .valve in each of said'by-pass ducts, means for preventing return flow ofrefrigerant from the intakes of said second and third compressors to said 'suction 1ine,means for preventing return flow of refrigerant from the intake of said third compressor to the intake of said second com pressor, means for preventing return flow of refrigerant from said condenser to the discharges f said first and said second compressors. means responsive to the refrigerant pressure in said suction line for opening the valve in said first by-pass duct when the refrigerant pressure is below a first predetermined 'value and closing saidvalve when the refrigerant pressure is above said rst predetermined value, and further means responsive to the refrigerant pressure in said suction line for closing the valve in said second by-pass duct when said refrigerant pressure is above a second predetermined value lower than said'rst predetermined value and for open-V ing said valve when said refrigerant pressure is CERTIFICATE op' coRREcTI'oN. Patent No. 518,518.1 hay 1+, 19h5- I I Emma LoDwIG.

I It is hereby certified'thgi: error eppears in the printed specification of the above numbered Vpaigeni: requiring correction as follows: Phge 2, first Parent should be ifea@ with this Eon-action therein thatthe sane my con-'- fomV to the reordoj thevhonse 1n- .the Patent Office.

Signed n .nd sealedl this thday of June, A. D. 1914.3.

Henry van Arsdale, x (Soll) Acting Commisnoner o1?A Patents. 

